What is the International Standard Atmosphere (ISA) and why does it matter for pilots?

The International Standard Atmosphere (ISA) is a defined set of standard conditions used as a reference for aviation calculations. ISA values at mean sea level: temperature 15°C, pressure 1013.25 hPa (29.92 inHg), density 1.225 kg/m³, temperature lapse rate 1.98°C per 1000 ft (approximately 2°C per 1000 ft) in the troposphere, and tropopause at 36,090 ft (11 km) where temperature stabilises at -56.5°C. Pilots use ISA to calculate density altitude, true altitude corrections, engine performance, and altimeter settings. Departures from ISA (ISA+ or ISA-) directly affect performance calculations — high temperatures reduce engine thrust and lift, while cold temperatures mean the aircraft is lower than the altimeter indicates.

What are the main types of aircraft icing and how do they differ?

The main types of aircraft icing are: Rime ice — forms in stratiform cloud at temperatures between 0°C and -20°C. It is white, opaque, and rough-textured, forming quickly as supercooled droplets freeze on contact. It is relatively light but disrupts airflow significantly. Clear ice (glaze ice) — forms in cumuliform cloud or freezing rain at temperatures between 0°C and -10°C. It is transparent, smooth, dense and very heavy. It follows the contours of the aircraft surface, making it difficult to see and hard to remove. It is the most dangerous form of icing. Mixed ice — a combination of rime and clear ice, typically in clouds containing a mixture of large and small water droplets. Additionally, frost can form on the ground in cold conditions, and must be removed before flight as even a thin layer significantly degrades lift.

What are the hazards of thunderstorms to aircraft?

Thunderstorms present multiple simultaneous hazards to aircraft: Severe turbulence — especially in and below cumulonimbus clouds; standing wave turbulence can extend well outside the cloud. Wind shear — sudden changes in wind speed and direction, especially dangerous during approach and departure. Microbursts — intense downdraughts that can cause severe performance loss and loss of control at low altitude. Lightning — can cause structural damage, temporary blindness, avionics disruption and fuel ignition. Hail — can occur well outside the visible cloud boundary and causes serious structural damage. Icing — rapid heavy icing in the mixed and supercooled water zones. Tornadoes and waterspouts. The recommended avoidance distance is 20 nm from the nearest edge of a visible cumulonimbus cell.

How do you decode a METAR for your ATPL exam?

A METAR is read in a standard sequence: (1) METAR or SPECI — type; (2) ICAO airport code; (3) Date/time in UTC (DDHHMMz); (4) AUTO if automated; (5) Wind — direction/speed/gusts in degrees true and knots (e.g. 27015KT); (6) Visibility in metres; (7) RVR — runway visual range if below 1500m; (8) Weather — present weather descriptors (RA=rain, SN=snow, FG=fog, TS=thunderstorm, etc.); (9) Cloud — sky condition in oktas and height in hundreds of feet (FEW=1-2, SCT=3-4, BKN=5-7, OVC=8; CB or TCU suffix); (10) Temperature and dewpoint in °C; (11) QNH in hPa; (12) TEMPO, BECMG or trend information.

What causes radiation fog and advection fog?

Radiation fog forms over land on calm, clear nights when longwave radiation cools the surface below the dewpoint temperature. Key conditions: light wind (0-5 kt) to allow slight mixing without dispersing the fog, high relative humidity, clear skies allowing rapid cooling, and a long night. It typically clears after sunrise as surface heating evaporates it from the base upward. Advection fog forms when warm moist air moves over a cooler surface (land or sea). Unlike radiation fog it requires wind to transport the air mass, and can persist in strong winds. Sea fog is a common type of advection fog and can be very persistent. The key difference: radiation fog is a still-air, nighttime/early-morning phenomenon; advection fog can occur at any time and in significant wind.

ATPL Theory — Meteorology

Master Aviation Weather for your ATPL theory exam

Free practice questions covering the atmosphere, clouds, fronts, icing, thunderstorms, METAR/TAF decoding, SIGMET, jet streams and ISA — with detailed explanations for every answer.

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10 Topic areas

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Syllabus Coverage

Topics covered in this question bank

All major Meteorology topic areas from the EASA ATPL syllabus, from basic atmospheric physics to operational weather products.

⊙The Atmosphere & ISA

⊙Clouds & Precipitation

⊙Air Masses & Fronts

⊙Pressure Systems & Wind

⊙Jet Streams & Global Circulation

⊙Visibility & Fog

⊙Aircraft Icing Types & Effects

⊙Thunderstorms & Hazards

⊙METAR, TAF & SIGMET

⊙Weather Radar & Satellite

Key Reference — International Standard Atmosphere

ISA values pilots must know for the exam

The International Standard Atmosphere (ISA) is defined by ICAO and used as the reference condition for all aviation performance calculations. These values appear directly in exam questions and must be memorised.

Parameter	ISA MSL Value	Notes
Temperature	15°C (288.15 K)	Lapse rate: 1.98°C / 1000 ft (~2°C / 1000 ft) in troposphere
Pressure	1013.25 hPa (29.92 inHg)	Halves approximately every 18,000 ft
Density	1.225 kg/m³	Decreases with altitude; affects lift, drag and engine performance
Tropopause	36,090 ft (11 km)	Temperature stabilises at −56.5°C above this level
Speed of Sound	661 kt at MSL	Decreases with temperature; ~573 kt at tropopause

EASA Exam Format

What to expect on the real exam

Meteorology is one of the broadest ATPL subjects, covering both theoretical concepts (ISA, lapse rates, frontal theory) and operational knowledge (METAR/TAF decoding, SIGMET criteria, icing avoidance). Questions range from straightforward recall to scenario-based weather interpretation. Icing types, thunderstorm hazards and fog formation are consistently high-frequency topics.

~84

Questions in exam

75%

Pass mark required

120 min

Exam duration

About This Subject

Why Meteorology is central to flight safety

Weather remains one of the primary causal factors in aviation accidents. The ability to interpret forecasts, recognise hazardous conditions, understand frontal systems and know the limits of weather avoidance is a fundamental professional pilot skill. The ATPL Meteorology paper tests both theory and practical weather product interpretation.

Questions are aligned with the EASA ATPL syllabus and cover the full range from basic atmospheric physics through to operational METAR/TAF reading, SIGMET interpretation and in-flight weather decision-making.

Sample Question

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Meteorology — Sample Question

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